Elevated MeHg Concentrations Research Articles

Ecohydrology of two northern Wisconsin bogs

AbstractThe dynamics of water and solutes were investigated in two northern bog ponds using sensor networks and discrete water samples. Embedded sensors monitored water level (S), precipitation (P), evaporation (E), water temperature (T) and specific conductivity (SC) in the peatlands and encircled ponds at 30 min time intervals from 2009 to 2015. Pond water chemistry was monitored seasonally from 2000 to 2020. Daily hydrographs and water budgets indicated that both bogs are ombrotrophic systems, perched above the local water table. Although the predominant flowpath for liquid water was precipitation → pond → peatland → underlying glacial deposits, evaporation accounted for 70% to 90% of water losses. High dissolved organic matter (DOM) in the ponds resulted from transient reversals of flowpath and from molecular diffusion across the peatland/pond interface (a tea bag effect). DOM of peatland origin dominated pond water chemistry, regulating the concentration of important metals, major nutrients and the acid–base status of both bog ponds. Elevated concentrations of Fe, Hg and MeHg in the ponds reflected ligand binding by DOM. The formation of DOM‐Fe‐PO4 complexes likely accounted for >3‐fold higher P concentration relative to nearby clearwater lakes. Linear regression of dissolved organic carbon (DOC) against the anion deficit indicated that DOM contributed up to 6.6 mEq of strong acid per gramme carbon in pond waters. Winter maxima in the seasonal cycles of DOC, Ca, Mg, N, P, Hg and MeHg in both bog ponds were attributable, in large part, to salting out during ice formation. We conclude that multiple methods are needed to understand the dynamics of water and solutes in bog ecosystems.

Mercury and methylmercury in Hg-contaminated paddy soil and their uptake in rice as regulated by DOM from different agricultural sources.

In this study, from the perspectives of structural and compositional variations of soil-dissolved organic matter (DOM), we explored the effects of agricultural DOM inputs on methylmercury (MeHg) accumulation in the soil and mercury (Hg) bioaccumulation in rice grains. Pot experiments with the addition of DOMs from maize straw (MaS), rape straw (RaS), rice straw (RiS), composted rice straw (CRiS), cow dung (CD), and composted cow dung (CCD) were then conducted. Results showed that, relative to the control, the DOM amendment from each agricultural source elevated MeHg concentrations in the soil, with an increase of 18-227%, but only parts of DOMs elevated total dissolved Hg (DHg) and MeHg (DMeHg) concentrations in pore water. Among all DOM species, RiS, CRiS, and CCD significantly increased total Hg (THg) and MeHg contents in rice grains by 34-64% and 32-118%, respectively. Compared with RiS, THg and MeHg contents in rice grains in the CRiS treatment decreased slightly, which was consistent with the distributions of DHg and DMeHg concentrations in pore water and the aromaticity variation of soil DOM. In contrast, the CCD input significantly enhanced the enrichment of THg and MeHg in rice grains relative to CD because it significantly reduced the humification of soil DOM at all rice-growing stages while increasing the low-molecular-weight fractions in soil DOM. The THg and MeHg contents in the rice grains were significantly lower treated by RaS than those by MaS and RiS, which may be related to the higher sulfur-containing compounds such as sulfate and cysteine in rape straw or its DOM solution. Overall, DOM amendment from different agricultural sources resulted in significantly discriminative effects on the MeHg accumulation in soil and Hg enrichment in rice in the Hg-contaminated paddy field by shaping soil DOM properties.

Organo-mercury species in a polluted agricultural flood plain: Combining speciation methods and polymerase chain reaction to investigate pathways of contamination

The analysis of organic mercury (Hg) species in polluted soils is a necessary tool to assess the environmental risk(s) of mercury in contaminated legacy sites. The artificial formation of monomethylmercury (MeHg) during soil extraction and/or analysis is a well-known limitation and is especially relevant in highly polluted areas where MeHg/Hg ratios are notoriously low. Although this has been known for almost 30 years, the thorough characterisation of artificial formation rates is rarely a part of the method development in scientific literature. Here we present the application of two separate procedures (inorganic Hg (iHg) spiking and double-spike isotope dilution analyses (DSIDA)) to determine and correct for artificial Hg methylation in MeHg-selective acid-leaching/organic solvent extraction procedure. Subsequently, we combined corrected MeHg and ethylmercury (EtHg) measurements with PCR amplification of hgcA genes to distinguish between naturally formed MeHg from primary deposited MeHg in soils from a legacy site in a Swiss mountain valley. We found the DSIDA procedure incompatible with the organomercury selective extraction method due to the quantitative removal of iHg. Methylation factors from iHg spiking were in the range of (0.0075 ± 0.0001%) and were consistent across soils and sediment matrices. Further, we suggest that MeHg was deposited and not formed in-situ in two out of three studied locations. Our line of evidence consists of 1) the concomitant detection of EtHg, 2) the elevated MeHg concentrations (up to 4.84 μg kg−1), and 3) the absence of hgcA genes at these locations. The combination of Hg speciation and methylation gene (hgcA) abundance analyses are tools suited to assess Hg pollution pathways at Hg legacy sites.

Experiments revealing the formation of refractory methylmercury pools in natural sediments and soils

Methylation and demethylation of mercury (Hg) are well recognized as processes controlling the concentrations of monomethylmercury (MeHg) in natural environments, and thus the pool of Hg available for biological uptake. In addition, studies have indicated the potential role of refractory MeHg pools (not readily available for demethylation) on the pool of MeHg in, for example, sediments and soils. These studies, however, remain scarce and often the role of refractory MeHg pools is overlooked. Here, we have conducted incubation experiments aiming to quantify refractory MeHg pools in contrasting environments. In our study, sediments (from lakes and brackish seawater sites) and soils (from forests and marshes) were incubated with isotopically enriched Hg tracers (Me201Hg and 198Hg) for up to 6 weeks. To follow the potential formation of refractory MeHg pools, %MeHg (fraction of Hg occurring as MeHg) after the first week of incubation for the added 198Hg and Me201Hg tracers, and ambient Hg was compared. The high %MeHg for the 198Hg tracer compared to the %MeHg of ambient Hg suggests a higher initial availability of added 198Hg in comparison to the ambient Hg in the sediments. For the soils, low %MeHg for the 198Hg tracer suggests low Hg methylation rates. The discrepancy observed between the sediments and soils can be explained by a higher availability of inorganic Hg in the sediments, as suggested by the Hg thermal fractional analysis conducted. The %MeHg steady state for the added Me201Hg tracer remained high (>17%) throughout the experiment, suggesting refractory pools of MeHg to be built-up in all tested sediments and soils. Together, the %MeHg for the added Hg tracers demonstrate that a significant fraction of the MeHg produced in sediments and soils is sequestered into refractory pools not readily available for demethylation. Furthermore, these results show that conditions favoring net methylation in sediments and soil could result in elevated concentrations of MeHg for a significant amount of time (months) even if the conditions favoring Hg net methylation are only temporary.

High concentrations of HgS, MeHg and toxic gas emissions in thermally affected waste dumps from hard coal mining in Poland

This study aims to provide numerous environmental research approaches to understand the formation of mineral and organic mercury compounds in self-heating coal waste dumps of the Upper Silesian Coal Basin (USCB). The results are combined with environmental and health risk assessments. The mineralogy comprised accessory minerals in the fine fraction of thermally affected waste, i.e., Hg sulfides, most likely cinnabar or metacinnabar. Moreover, other metals, e.g., Pb, Zn and Cu, were found as sulfide forms. Apart from Hg, the ICP-ES/MS data confirmed the high content of Mn, Zn, Pb, Hg, Cr and Ba in these wastes. The high concentration of available Hg resulted in elevated MeHg concentrations in the dumps. There were no correlations or trends between MeHg concentrations and elemental Hg, TS, TOC, and pH. Furthermore, we did not detect microbial genes responsible for Hg methylation. The organic compounds identified in waste and emitted gases, such as organic acids, or free methyl radicals, common in such burn environments, could be responsible for the formation of MeHg. The concentration levels of gases, e.g., benzene, formaldehyde, NH3, emitted by the vents, reached or surpassed acceptable levels numerous times. The potential ecological and human health risks of these dumps were moderate to very high due to the significant influence of the high Hg concentrations.

Impacts of water level fluctuations on mercury concentrations in hydropower reservoirs: A microcosm experiment

Hydropower generation, a renewable source of electricity, has been linked to elevated methylmercury (MeHg) concentrations in impoundments and aquatic biota. This study investigates the impact of water level fluctuations (WLF) on MeHg concentrations in water, sediment, and fish. Using a set of controlled microcosm experiments emulating the drawdown/refill dynamics and subsequent sediment exposure to air experienced in reservoirs, we demonstrate that less frequent WLFs, and/or increased exposure of sediment to air, can lead to elevated MeHg concentrations in sediment, and total mercury (THg) and MeHg concentrations in water. In examining the effects of WLF frequency (two-day, weekly, and monthly), the monthly treatment displayed the highest THg and MeHg water levels, while the weekly treatment was characterized by the highest MeHg levels in the sediment. Our work supports emerging evidence that longer duration between WLF creates a larger surface area of sediment exposed to air leading to conditions conducive to higher MeHg concentrations in sediments and water. In contrast, THg, MeHg, and fatty acid trends in fish were largely inconclusive characterized by similar among-treatment effects and minimal temporal variability over the course of our experiment. This result could partly be attributed to overall low mercury levels and simple “worm-forage fish” food web in our experiment. To elucidate the broader impacts of water fluctuations on aquatic chemistry and biota, other factors (e.g., longer WLF cycles, dissolved organic matter, temperature, more complex food webs) which modulate both methylation rates and food web dynamics must be considered.

Methylmercury Production and Degradation under Light and Dark Conditions in the Water Column of the Hells Canyon Reservoirs, USA.

Methylmercury (MeHg) is a highly toxic form of mercury that can bioaccumulate in fish tissue. Methylmercury is produced by anaerobic bacteria, many of which are also capable of MeHg degradation. In addition, demethylation in surface waters can occur via abiotic sunlight-mediated processes. The goal of the present study was to understand the relative importance of microbial Hg methylation/demethylation and abiotic photodemethylation that govern the mass of MeHg within an aquatic system. The study location was the Hells Canyon complex of 3 reservoirs on the Idaho-Oregon border, USA, that has fish consumption advisories as a result of elevated MeHg concentrations. Our study utilized stable isotope addition experiments to trace MeHg formation and degradation within the water column of the reservoirs to understand the relative importance of these processes on the mass of MeHg using the Water Quality Analysis Simulation Program. The results showed that rates of MeHg production and degradation within the water column were relatively low (<0.07 d-1 ) but sufficient to account for most of the MeHg observed with the system. Most MeHg production within the water column appeared to occur in the spring when much of the water column was in the processes of becoming anoxic. In the surface waters, rates of photodemethylation were relatively large (up to -0.25 d-1 ) but quickly decreased at depths >0.5 m below the surface. These results can be used to identify the relative importance of MeHg processes that can help guide reservoir management decisions. Environ Toxicol Chem 2021;40:1829-1839. © 2021 SETAC. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Historic contamination alters mercury sources and cycling in temperate estuaries relative to uncontaminated sites

Mercury (Hg) is a global and persistent pollutant which can be methylated to more toxic forms (methylmercury; MeHg) in natural systems. Both forms pose a health risk to humans and wildlife, and exposure often begins in aquatic environments. Therefore, quantifying aquatic concentrations and identifying source pathways is important for understanding biotic exposure. In this study, data from estuaries in the Northeast United States were combined to evaluate how point source contamination impacts the concentration and source dynamics of water column total and MeHg with an emphasis on sediment versus non-sediment sources. Partial least squares regression models were implemented to identify a set of variables most related to water column MeHg and total Hg (HgT) across the estuaries. The main findings suggest that contaminated sites have strong internal recycling of HgT that dominates over external inputs, and this leads to elevated concentrations of HgT and MeHg in the local water columns. However, HgT sources in uncontaminated estuarine systems have a strong connection to the local watershed with dissolved HgT linked to dissolved organic carbon, and particulate HgT linked to watershed land use and estuarine mixing. There was little correlative evidence that water column MeHg concentrations were linked to sediment in such systems, but unlike HgT, the concentrations were also not clearly linked to the watershed. Instead, in situ methylation of dissolved water column HgT appeared to dominate the MeHg source pathway. The results suggest that Hg point-source contaminated sites should be considered independently from non-contaminated sites in terms of management, and that land use plays an important indirect role in coastal MeHg dynamics.

Impact of multiple drying and rewetting events on biochar amendments for Hg stabilization in floodplain soil from South River, VA

Frequent drying and rewetting due to flooding/precipitation and drainage events in floodplains induces changes in biogeochemical conditions that may influence the effectiveness of in situ Hg stabilization using biochars as soil amendments. This study evaluated two selected biochars anaerobic digestate (DIG) and sulfurized hardwood (MOAK)) as potential amendment materials in moderately reduced floodplain soil under repeated drying and rewetting events using a modified humidity cell protocol. Enhanced release of filter-passing (0.45-μm) total Hg (THg) and MeHg was observed at early times. Elevated concentrations of 0.45-μm THg were associated with DOC and Mn in sediment control and biochar-amended systems. Elevated concentrations of MeHg were associated with Mn in the MOAK-amended system. Thereafter, decreases in 0.45-μm (up to 57%) and unfiltered THg (up to 93%) were observed. As wetting and drying events continued, decreases in pH and alkalinity as well as increases in SO42− (up to 796 mg L−1) and Ca (up to 215 mg L−1) were observed in the MOAK-amended systems with the microbial community shifted towards sulfur-oxidizing bacteria, indicating microbially-driven oxidation of MOAK. Although results of S K-edge X-ray absorption near edge structure (XANES) analysis suggest polysulfur is the predominant S phase in both MOAK- and DIG-amended systems, microbially-driven oxidation of DIG was not observed. Polysulfur in MOAK from the sulfurization process is more bioavailable to sulfur oxidizing communities than in DIG under the repeated drying and wetting conditions. Results of this study suggest biogeochemical conditions as well as biochar properties should be considered when planning full-scale field applications.

Relationship Between Methylmercury Contamination and Proportion of Aquatic and Terrestrial Prey in Diets of Shoreline Spiders

Terrestrial organisms such as shoreline spiders that consume prey from aquatic food webs can be contaminated with methylmercury (MeHg). However, no studies have examined the relationship between MeHg contamination of shoreline spider taxa and the proportion of aquatic and terrestrial prey in their diets. The present study had 2 objectives: 1) determine concentrations of MeHg in 7 taxa of shoreline spiders, and 2) assess the relationship between concentrations of MeHg in spiders and the proportion of aquatic and terrestrial prey in spider diets. We collected shoreline spiders, emergent aquatic insects, and terrestrial insects from in and around 10 experimental ponds. Methylmercury concentrations were greatest in spiders, intermediate in aquatic insects, and lowest in terrestrial insects. The elevated MeHg concentrations in spiders indicate that they were feeding, at least in part, on emergent aquatic insects. However, variability in MeHg concentration observed among spider taxa suggested that the proportion of aquatic and terrestrial prey in spider diets likely varied among taxa. We estimated the proportion of aquatic and terrestrial prey in the diet of each spider taxon from the nitrogen (δ15 N) and carbon (δ13 C) isotope values of spiders and their potential aquatic and terrestrial prey items. The median proportion of aquatic prey in spider diets varied by almost 2-fold, and MeHg concentrations in shoreline spiders were strongly correlated with the proportion of aquatic prey in their diet. In the present study, we demonstrate for the first time that the degree of connectivity to aquatic food webs determines MeHg contamination of shoreline spiders. Environ Toxicol Chem 2019;38:2503-2508. © 2019 SETAC.

Newly deposited atmospheric mercury in a simulated rice ecosystem in an active mercury mining region: High loading, accumulation, and availability

Mercury (Hg) mining activities are an important anthropogenic source of atmospheric Hg. The Xunyang Hg mine located in Shaanxi Province is the largest active Hg producing centre in China. To understand the biogeochemical processes of atmospheric Hg through Hg mining activities, six groups of experimental pots were carefully designed to investigate the effect of Hg mining activities on Hg contamination from atmospheric deposition in the local surface soils. Based on the variations of Hg in the soil from the experimental pots, the deposition flux and loading of Hg in the Xunyang Hg mining district were investigated. The results showed that the average concentration of total gaseous mercury (TGM) as high as 193 ± 122 ng m-3 was observed in the ambient air, which was orders of magnitude higher than that in remote areas. The average deposition flux and annual loading of atmospheric Hg were 72 mg m-2 y-1 and 10 t y-1, respectively. The dominant atmospheric Hg deposition is within a distance range of 6.0-12 km from the Hg retorting facility, accounting for approximately 85% of the total Hg loading. After 14 months of exposure, total mercury (THg) concentrations in the soil from the experimental pots increased 0.35-9.5 times, and the highest concentrations of methylmercury (MeHg) (3.7 ± 2.9 μg kg-1) in soil were observed in February. Concentrations as high as 643 μg kg-1 THg and 13 μg kg-1 MeHg in rice were observed in the second experimental year. Elevated concentrations of both THg and MeHg in rice indicated that the newly deposited atmospheric Hg was bioavailable, readily methylated, and taken up by rice, suggesting that the ongoing Hg mining activities cause serious Hg contamination in the soil-rice ecosystem and posed a threat to local residents in the Xunyang Hg mining area.

Nutrients mediate the effects of temperature on methylmercury concentrations in freshwater zooplankton

Methylmercury (MeHg) bioaccumulation in freshwater aquatic systems is impacted by anthropogenic stressors, including climate change and nutrient enrichment. The goal of this study was to determine how warmer water temperatures and excess nutrients would alter zooplankton communities and phytoplankton concentrations, and whether those changes would in turn increase or decrease MeHg concentrations in freshwater zooplankton. To test this, we employed a 2 × 2 factorial experimental design with nutrient and temperature treatments. Mesocosms were filled with ambient water and plankton from Cottage Grove Reservoir, Oregon, U.S.A., a waterbody that has experienced decades of elevated MeHg concentrations and corresponding fish consumption advisories due to run-off from Black Butte Mine tailings, located within the watershed. Treatment combinations of warmer temperature (increased by 0.7 °C), nutrient addition (a single pulse of 10× ambient concentrations of nitrogen and phosphorous), control, and a combination of temperature and nutrients were applied to mesocosms. The individual treatments altered phytoplankton densities and community structure, but alone the effects on MeHg concentrations were muted. Importantly, we found a significant interactive effect of nutrients and temperature: the nutrient addition appeared to buffer against increased MeHg concentrations associated with elevated temperature. However, there was variability in this response, which seems to be related to the abundance of Daphnia and edible phytoplankton. Nutrients at low temperature were associated with marginal increases (1.1×) in zooplankton MeHg. Our findings suggest that global change drivers that influence community composition and ecosystem energetics of both zooplankton and phytoplankton can alter MeHg pathways through food webs.

Spatial and taxonomic variation of mercury concentration in low trophic level fauna from the Mediterranean Sea.

Studies of mercury (Hg) in the Mediterranean Sea have focused on pollution sources, air-sea mercury exchange, abiotic mercury cycling, and seafood. Much less is known about methylmercury (MeHg) concentrations in the lower food web. Zooplankton and small fish were sampled from the neuston layer at both coastal and open sea stations in the Mediterranean Sea during three cruise campaigns undertaken in the fall of 2011 and the summers of 2012 and 2013. Zooplankton and small fish were sorted by morphospecies, and the most abundant taxa (e.g. euphausiids, isopods, hyperiid amphipods) analyzed for methylmercury (MeHg) concentration. Unfiltered water samples were taken during the 2011 and 2012 cruises and analyzed for MeHg concentration. Multiple taxa suggested elevated MeHg concentrations in the Tyrrhenian and Balearic Seas in comparison with more eastern and western stations in the Mediterranean Sea. Spatial variation in zooplankton MeHg concentration is positively correlated with single time point whole water MeHg concentration for euphausiids and mysids and negatively correlated with maximum chlorophyll a concentration for euphausiids, mysids, and "smelt" fish. Taxonomic variation in MeHg concentration appears driven by taxonomic grouping and feeding mode. Euphausiids, due to their abundance, relative larger size, importance as a food source for other fauna, and observed relationship with surface water MeHg are a good candidate biotic group to evaluate for use in monitoring the bioavailability of MeHg for trophic transfer in the Mediterranean and potentially globally.

Bioaccumulation characteristics of mercury in fish in the Three Gorges Reservoir, China

Newly constructed reservoirs were recognized as hotspot of mercury (Hg) methylation, and then methylmercury (MeHg) accumulation in food chains. The risk of elevated MeHg concentrations in fish is one of the most important concerns in newly constructed reservoirs. The Three Gorges Reservoir (TGR) is one of the largest reservoirs in the world. However, the distribution and bioaccumulation characteristics of Hg species within the food chains and its potential ecological risk in the TGR remain poorly understood. In this study, 264 fish individuals covering 18 species were collected from the TGR. Total mercury (THg) and MeHg concentrations in different organs (gill, heart, liver, muscle and swim bladder) of fish species were analyzed; the values of δ13C and δ15N in fish muscle were determined as well to reveal the biomagnification properties of Hg in food chains. Our results showed that concentrations of THg (0.5–272 ng g−1, w.w.) and MeHg (0.1–199 ng g−1, w.w.) in fish muscle from the TGR ubiquitously fall below the safe fish consumption limit on Hg recommended by WHO (500 ng g−1, w.w.) and the US-EPA Water Quality Criterion for MeHg (300 ng g−1, w.w.). The short food web jointly with the limited trophic magnification factor in the TGR explained the relatively low Hg concentrations in predators. Among the five fish organs, muscle represented the highest Hg concentrations, followed by heart, liver, swim bladder, and gill, suggesting that muscle has the highest ability to accumulate Hg compared to the other organs. More importantly, no discernible “reservoir effect” was observed in the TGR within the initial few years after impoundment due to its special eco-environment including: 1) neutral and slightly alkaline pH and low dissolved organic carbon of water, 2) less vegetation coverage in inundated areas, 3) simple food web.

Role of the floodplain lakes in the methylmercury distribution and exchanges with the Amazon River, Brazil

Seasonal variability of dissolved and particulate methylmercury (F-MeHg, P-MeHg) concentrations was studied in the waters of the Amazon River and its associated Curuai floodplain during hydrological year 2005–2006, to understand the MeHg exchanges between these aquatic systems. In the oxic white water lakes, with neutral pH, high F-MeHg and P-MeHg concentrations were measured during the rising water stage (0.70±0.37pmol/L, n=26) and flood peak (14.19±9.32pmol/g, n=7) respectively, when the Amazon River water discharge into the lakes was at its maximum. The lowest mean values were reported during the dry season (0.18±0.07pmol/L F-MeHg, n=10 and 1.35±1.24pmol/g P-MeHg, n=8), when water and suspended sediments were outflowing from the lakes into the River. In these lakes, the MeHg concentrations were associated to the aluminium and organic carbon/nitrogen changes. In the black water lakes, with acidic pH and reducing conditions, elevated MeHg concentrations were recorded (0.58±0.32pmol/L F-MeHg, n=16 and 19.82±15.13pmol/g P-MeHg, n=6), and correlated with the organic carbon and manganese concentrations. Elevated values of MeHg partition coefficient (4.87<Kd<5.08log (L/kg) indicate that MeHg is mainly transported associated with the particulate phase. The P-MeHg enrichment detected in all lakes suggests autochthonous MeHg inputs from the sediments into the water column. The MeHg mass balance showed that the Curuai floodplain is not the source of P-MeHg for the Amazon River.

Controls on the Variation of Methylmercury Concentration in Seagrass Bed Consumer Organisms of the Big Bend, Florida, USA

Mercury poses both ecosystem and human health concerns that are exacerbated by the production of methylmercury (CH3Hg+; MeHg), which is often associated with sulfate-reducing bacteria (SRB) in marine sediments. We examined the relationship between MeHg concentrations and reduced-sulfur stable isotope δ34S values in four coastal consumer organisms collected from northern and southern regions of Florida’s Big Bend seagrass meadows: pinfish (Lagodon rhomboides), pigfish (Orthopristis chrysoptera), black sea bass (Centropristis striata), and shrimp (Tozeuma carolinense and Tozeuma serratum). We tested the hypothesis that depletion in δ34S, which is an indicator of the extent of sulfate reduction within sediments of seagrass dominated systems, would be related to the concentration of MeHg in fauna within these systems. We found that as δ34S became more depleted, indicating more relative sulfate reduction in sediments, the MeHg concentrations increased significantly (r = 0.52; p < 0.001; total n = 157). Stable δ34S values ranged from 17.4 to 4.8‰ and MeHg concentrations from 37.3 to 832 ng g−1 dry weight (DW). Correlations between tissue MeHg and δ13C were also observed (r = 0.44; p < 0.001; total n = 122), indicating that variations in benthic/water column feeding could also be important in controlling the MeHg distributions in the selected fauna. In addition, we observed spatial differences in δ34S, δ13C, and δ15N isotopic values and MeHg concentrations of consumers. Fauna from the southern region of the study area were 13C enriched, 15N depleted, 34S depleted, and had elevated MeHg concentrations relative to their northern Big Bend counterparts.

Methylmercury Modulation in Amazon Rivers Linked to Basin Characteristics and Seasonal Flood-Pulse.

We investigated the impact of the seasonal inundation of wetlands on methylmercury (MeHg) concentration dynamics in the Amazon river system. We sampled 38 sites along the Solimões/Amazon and Negro rivers and their tributaries during distinct phases of the annual flood-pulse. MeHg dynamics in both basins was contrasted to provide insight into the factors controlling export of MeHg to the Amazon system. The export of MeHg by rivers was substantially higher during high-water in both basins since elevated MeHg concentrations and discharge occurred during this time. MeHg concentration was positively correlated to %flooded area upstream of the sampling site in the Solimões/Amazon Basin with the best correlation obtained using 100 km buffers instead of whole basin areas. The lower correlations obtained with the whole basin apparently reflected variable losses of MeHg exported from upstream wetlands due to demethylation, absorption, deposition, and degradation before reaching the sampling site. A similar correlation between %flooded area and MeHg concentrations was not observed in the Negro Basin probably due to the variable export of MeHg from poorly drained soils that are abundant in this basin but not consistently flooded.

Mercury risk in poultry in the Wanshan Mercury Mine, China

In this study, total mercury (THg) and methylmercury (MeHg) concentrations in muscles (leg and breast), organs (intestine, heart, stomach, liver) and blood were investigated for backyard chickens, ducks and geese of the Wanshan Mercury Mine, China. THg in poultry meat products range from 7.9 to 3917.1 ng/g, most of which exceeded the Chinese national standard limit for THg in meat (50 ng/g). Elevated MeHg concentrations (0.4–62.8 ng/g) were also observed in meat products, suggesting that poultry meat can be an important human MeHg exposure source. Ducks and geese showed higher Hg levels than chickens. For all poultry species, the highest Hg concentrations were observed in liver (THg: 23.2–3917.1 ng/g; MeHg: 7.1–62.8 ng/g) and blood (THg: 12.3–338.0 ng/g; MeHg: 1.4–17.6 ng/g). We estimated the Hg burdens in chickens (THg: 15.3–238.1 μg; MeHg: 2.2–15.6 μg), ducks (THg: 15.3–238.1 μg; MeHg: 3.5–14.7 μg) and geese (THg: 83.8–93.4 μg; MeHg: 15.4–29.7 μg). To not exceed the daily intake limit for THg (34.2 μg/day) and MeHg (6 μg/day), we suggested that the maximum amount (g) for chicken leg, breast, heart, stomach, intestine, liver, and blood should be 1384, 1498, 2315, 1214, 1081, 257, and 717, respectively; the maximum amount (g) for duck leg, breast, heart, stomach, intestine, liver, and blood should be 750, 1041, 986, 858, 752, 134, and 573, respectively; and the maximum amount (g) for goose leg, breast, heart, stomach, intestine, liver, and blood should be 941, 1051, 1040, 1131, 964, 137, and 562, respectively.

Mercury methylation in paddy soil: source and distribution of mercury species at a Hg mining area, Guizhou Province, China

Abstract. Rice paddy plantation is the dominant agricultural land use throughout Asia. Rice paddy fields have been identified as important sites for methylmercury (MeHg) production in the terrestrial ecosystem and a primary pathway of MeHg exposure to humans in mercury (Hg) mining areas. We compared the source and distribution of Hg species in different compartments of the rice paddy during a complete rice-growing season at two different typical Hg-contaminated mining sites in Guizhou province, China: an abandoned site with a high Hg concentration in soil but a low concentration in the atmosphere and a current-day artisanal site with a low concentration in soil but a high concentration in the atmosphere. Our results showed that the flux of new Hg to the ecosystem from irrigation and atmospheric deposition was insignificant relative to the pool of old Hg in soil; the dominant source of MeHg to paddy soil is in situ methylation of inorganic Hg (IHg). Elevated MeHg concentrations and the high proportion of Hg as MeHg in paddy water and the surface soil layer at the artisanal site demonstrated active Hg methylation at this site only. We propose that the in situ production of MeHg in paddy water and surface soil is dependent on elevated Hg in the atmosphere and the consequential deposition of new Hg into a low-pH anoxic geochemical system. The absence of depth-dependent variability in the MeHg concentration in soil cores collected from the abandoned Hg mining site, consistent with the low concentration of Hg in the atmosphere and high pH of the paddy water and irrigation water, suggested that net production of MeHg at this site was limited. We propose that the concentration of Hg in ambient air is an indicator for the risk of MeHg accumulation in paddy rice.

Mercury and methylmercury in a reservoir during seasonal variation in hydrology and circulation

This study explored the effect of seasonally varying inflow hydrology and circulation on total mercury (Hg) and methylmercury (MeHg) in the water column of a warm, temperate reservoir. The study site in Grand Lake, Oklahoma, is >80 km long and usually 36 m deep at its dam. During this study, aside from 2 large, storm-driven springtime events, drought caused low inflows. During maximal summer stratification, the surface mixed layer was 8–11 m thick, water below was anoxic, and inflows entered as an interflow. Total Hg in filtered samples was <4 ng/L with an interquartile range of 0.27–0.96 ng/L. Highest concentrations occurred in spring and summer, and lowest concentrations followed overturn. In filtered samples, MeHg was ≤0.62 ng/L with an interquartile range of 0.01–0.08 ng/L. Highest concentrations of MeHg occurred in some large inflows and in anoxic summertime bottom water in specific locations where total suspended sediment was also elevated. During this study, Hg concentrations in Grand Lake were driven by inflow hydrology when inflows were high. During low inflows, biogeochemistry controlled the enrichment of MeHg in specific locations of the anoxic bottom water and sequestered both Hg species in autumn. This sequestration suggests that Grand Lake decreases watershed-derived Hg pollution downstream. Occurrence of elevated Hg and MeHg concentrations primarily in large inflows and anoxic bottom water suggests that exposure of biota to Hg species is greatest when floods enter the reservoir because Hg enrichment in deep, anoxic water does not coincide with the habitat of most fauna.